The present invention relates to a digital carrier signal demodulation circuit for use in carrier digital transmission in which the carrier is modulated with a plurality of base band signals or pulses.
For the transmission of digital information there have been devised and used various modulation methods such as ASK (Amplitude Shift Keying), PSK (Phase Shift Keying) and FSK (Frequency Shift Keying). Among them the four-phase shift keying method or 4 PSK is most frequently used, but it has a distinct defect that the information transmittable by this method is only two bits per symbol. The four-phase shift keying method is therefore not necessarily an efficient modulation method.
One of the methods for improving the efficiency of the four-phase shift keying method is to increase a number of modulation phases to 8,16 and so on, but this improvement results in the rapid degradation in margin for noise as the number of modulation phases is increased. One of the methods for improving the efficiency of the amplitude modulation method is also to increase the number of levels used to 4,8 and so on, but this method also results in the rapid degradation in margin for noise as the number of levels used is increased.
The methods for overcoming the above defects by simultaneously modulating both the amplitude and phase are disclosed in, for instance, U.S. Pat. Nos. 3,619,503 and 3,805,191. These methods called Amplitude and Phase Shift Keying or APSK are very advantageous over the conventional amplitude and phase modulation methods in that the phase-amplitude plane is utilized in a very efficient manner so that they are very excellent digital information transmission methods.
In amplitude and phase shift keying, a reference signal whose phase is in exact synchronism with that of the carrier produced in a transmitter must be produced in a receiver for demodulating the received signal. As compared with the phase modulation, it is extremely difficult to extract the reference signal from the received signal except a special case of amplitude and phase shift keying. To carry out the conventional amplitude and phase shift keying methods, logic circuits and digital-to-analog converts with complicated structure must be provided so that the high speed signal transmission cannot be attained. Mainly because of the reasons described above, even though the amplitude and phase shift keying methods provide an excellent communication system, so far they have not been utilized in practice yet. Furthermore, high-speed amplitude and phase shift keying has never been utilized in practice.